In a collision which of the mechanical forces is involved explain briefly

In a collision, there'll always be two mechanical force involved. One is the force of acceleration and the other is the frictional force. In an event of a collision, the two colliding bodies will experience change in acceleration, one who has the larger momentum will retain its direction of acceleration, all the while both body will experience another which is frictional force.

it is given that trillions of microbes make up to 1% to 3% of the body mass.

let's take average of this percentage: 2%

consider the average mass of the human body as 70 kg.

then, trillions of microbes would have mass  

therefore, 100 trillion micro-organisms would weigh:

this is an approximate value and could change depending on the mass of the human body and percentage of the microbes present in it.

average mass per microorganism =